The present invention relates to novel electrophotographic elements containing photoconductive layers comprising a polyester resin matrix having photoconductors dispersed therein. More particularly, the present invention relates to a new class of polyester resins which are highly advantageous for use as matrix polymers in electrophotographic elements, producing a significant improvement in the photographic speed, and in many cases, the dark decay properties of the system. In addition, electrophotographic elements containing matrices of these polymers are readily developed by fusing techniques, such as for example flash fusing development techniques.
Electrophotographic elements containing photoconductive layers of polyester resin matrices and dispersed photoconductors are well known in the art. See, for example, U.S. Pat. Nos. 3,703,371 and 3,703,372. In such materials, a sensitizer is frequently employed together with the photoconductor. In systems of this type, the electrostatic latent image photographic speed depends upon the sensitization and carrier generation efficiency as well as the carrier transport rate, i.e., it is theoretically possible to have a very light sensitive film that affords little or no latent image due to poor carrier transport. The matrix can thus affect the useful photographic speed for such film by determining the magnitude of the final voltage drop as well as the time to achieve it. The polyester resins heretofore known in the art have generally contributed to the photographic speed only in those few cases where the polyester resin forms a co-crystalline complex with the sensitizer molecule. As a result, the photographic speed of films based on many of the known polyester resin matrices has been less than desirable.
Moreover, electrophotographic elements containing polyester matrices frequently possess less than desirable dark decay properties, i.e. the ability of the electrophotographic element to retain a stable charge level on the film surface in the dark. As a result of charge decay, in electrophotographic elements having poor dark decay properties, the image quality is dependent on the time elapsed between charging and exposure as well as exposure and development, a condition which is commercially undesirable.
In addition, the polyester matrices heretofore employed in the prior art have possessed poor fusibility, and electrophotographic elements containing matrices of these polyesters have been especially difficult to fix by the flash fusing technique. As is well known to those skilled in the art, flash fusing techniques comprise a highly attractive method of image fixation due to both its convenience and the high level of image permanence resulting therefrom. As mentioned above, this type of image fixation technique has been difficult to achieve with electrophotographic elements containing the polyester matrices which have heretofore been employed in the art.
Polyester resins useful as photoconductive matrices in electrophotographic films must not only possess excellent dielectric properties, but must also have a glass transition temperature (Tg) sufficiently high that the formulated film does not exhibit a blocking tendency when in contact with itself or another surface. In addition, in order to achieve a desirable level of physical properties with respect to fracture radius, adhesion, crinkle phenomena, etc., it is usually necessary to employ polyester resins having moderately high molecular weights, e.g. above 20,000 molecular weight units. This, however, results in a film that does not have the proper rheology to flow well under the conditions necessary to the fixation of an electrophotographic image having satisfactory image durability by fusing techniques. The obvious compromise, the use of very low molecular weight polymers, which improving the fusibility of the film, requires a sacrifice in the other desirable physical properties of the film discussed above. In addition, the use of lower molecular weight resins can result in a serious loss of dark decay and photographic speed characteristics.
The compromise referred to above, moreover, is not without limitation, and the molecular weight of the polyester resin must be sufficiently high to provide the minimum level of glass transition temperature, adhesion, fracture radius, etc. necessary to the preparation of useful film. The end result is that the polyester resins heretofore employed as photoconductor matrices in electrophotographic elements have exhibited adequate physical properties while at the same time being difficultly fusible with electrophotographic toner particles by the flash fusing technique. In addition, as discussed above, electrophotographic films containing matrices of these resins have generally possessed less than desirable photographic speed and dark decay properties. Accordingly, the development of a new class of polyester resins which possess a molecular weight sufficient to maximize physical properties, photographic speed and dark decay properties, while at the same time being readily fusible with electrophotographic toners by flash fusing or other similar techniques, would be a highly desirable contribution to the art.